Tuesday 27 November 2012

Soft tissue preservation in Linguloid Brachiopods from the Early Ordovician Fenxiang Formation of Hubei Province, China.

Brachiopods (or Lampshells) superficially resemble Bivalve Molluscs, though they are not closely related. They have a filter feeding apparatus called a lophophore, unlike anything found in any Mollusc, but also found in Bryozoans and Phoronid Worms. This is encased with in a shell with two valves, each symmetrical about a midline, but not necessarily the same as each other, along with the rest of the organs of the body; there is typically remarkably little flesh to a Brachiopod compared to a Mollusc with a shell the same size. However one group of Brachiopods, the Linguloids, which have a long, fleshy, worm-like body with a shell enclosing only the head-area; these Brachiopods have been compared to a Phoronid Worm with a partial shell, and are considered to be the earliest group of Brachiopods; they appear in the fossil record early in the Cambrian, and essentially unchanged forms still exist today.

Modern Linguloid Brachiopods. Suny Cortland.

In a paper published in the journal Acta Palaeontologica Polonica on 1 October 2012, Andrzej Baliński of the Instytut Paleobiologii at the Polish Academy of Sciences and Yuanlin Sun of the Key Laboratory of Orogenic Belts and Crustal Evolution at the School of Earth and Space Sciences at Peking University describe the occurrence of Linguloid Brachiopods with preserved soft-tissue tails from the early Ordovician Fenxiang Formation of Hubei Province, China.

This is not the oldest such occurrence, Linguloid Brachiopods have been found with their tails preserved in both the Burgess Shale and Chengjang Faunas from the Cambrian, but it is still a significant find. The Cambrian specimens are not universally accepted as having been burrowing animals; some scientists believe that in the rather different ecosystems of the Early Cambrian they may not have needed to burrow; there is certainly no clear evidence of burrowing by any form of animal from sediments of this time. The Ordovician is a different matter, burrowing lifestyles had definitely evolved by this time, and there was no shortage of predators available to attack an exposed Linguloid Brachiopod on the surface.

The method of preservation is also different to that seen in the Cambrian fossils, which are preserved as two-dimensional aluminosilicate or degraded organic carbon films in shales (laminated mudstones), a method of preservation extremely rare after the Early Cambrian due to bioturbation of marine sediments by burrowing animals. In contrast these Ordovician fossils are preserved as three-dimensional mineralized body-fossils. In these specimens the mineralized soft tissue is a bright-red colour, due to the oxidation of iron minerals. The soft tissues are thought to have initially been replaced with hydrogen sulphide (H₂S) by sulphur-reducing bacteria in an anaerobic (oxygen-lacking environment), this Hydrogen Sulphide is then thought to have reacted with iron from the surrounding sediments, giving it its distinctive colour.

Preserved Linguloid Brachiopod from the lower Ordovician Fenxiang Formation. Scale bar is 5 mm. Baliński & Sun (2012).

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